agent: Fix panic caused by polling ended stream (#46839)

tidely created 

Closes #ISSUE

Reproduction steps:

- Zed main branch
- Use Ollama (likely provider agnostic) in the agent panel with the
following prompt: "Explore the codebase with subagents"
- Panic

```
thread 'main' (36268) panicked at C:\Users\username\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\futures-util-0.3.31\src\stream\unfold.rs:108:21:
Unfold must not be polled after it returned `Poll::Ready(None)`
```

Following the stack trace we get to `Thread::run_turn_internal`. I
believe the panic happens in the following code which was introduced in
#46802

```rust
// Collect all immediately available events to process as a batch
let mut batch = vec![first_event];
while let Some(event) = events.next().now_or_never().flatten() {
    batch.push(event);
}
```

Both `Option`s get flattened, however the inner `Option` represents the
end of the stream, after which polling the stream using `.next()` will
result in a panic.

We could fix the logic in this particular spot, but I believe the
simpler solution is to `.fuse()` the stream, which stops the stream from
panic'ing even after it has ended. This also prevents misuse in the
future.

The panic was introduces on main and did not land on a release yet, so
no release notes.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

Change summary

crates/agent/src/thread.rs | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

Detailed changes

crates/agent/src/thread.rs 🔗 

@@ -1533,8 +1533,8 @@ impl Thread {
             log::debug!("Calling model.stream_completion, attempt {}", attempt);
 
             let (mut events, mut error) = match model.stream_completion(request, cx).await {
-                Ok(events) => (events, None),
-                Err(err) => (stream::empty().boxed(), Some(err)),
+                Ok(events) => (events.fuse(), None),
+                Err(err) => (stream::empty().boxed().fuse(), Some(err)),
             };
             let mut tool_results = FuturesUnordered::new();
             let mut cancelled = false;